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2000 ARRS Executive Council Award I |
1 All authors: Department of Radiology, New York Presbyterian Hospital, Columbia University College of Physicians and Surgeons, Milstein Hospital Bldg., 2nd Fl., Fort Washington Ave., New York, NY 10032.
Received February 24, 2000;
accepted after revision June 5, 2000.
Address correspondence to K. M. Duwe.
Abstract
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MATERIALS AND METHODS. Patients who had undergone a combined CT angiographic and CT venographic protocol and sonography of the lower extremities within 1 week were identified. The final reports were evaluated for the presence or absence of deep venous thrombosis. Statistical measures for the identification of deep venous thrombosis with helical CT venography were calculated. In each true-positive case, the location of the thrombus identified with both techniques was compared. All false-positive and false-negative cases were reviewed to identify the reasons for the discrepancies.
RESULTS. Seventy-four patients were included. There were eight patients (11%) with true-positive findings, 61 patients (82%) with true-negative findings, four patients (5%) with false-positive findings, and one patient (1%) with a false-negative finding. When comparing helical CT venography with sonography for the detection of lower extremity deep venous thrombosis, the sensitivity measured 89%; specificity, 94%; positive predictive value, 67%; negative predictive value, 98%; and accuracy, 93%. Of the eight true-positive cases, five had sites of thrombus that were in agreement on both CT venography and sonography. Of the five discordant cases, four were false-positives and one was a false-negative. Possible explanations for all discrepancies were identified.
CONCLUSION. Compared with sonography, CT venography had a 93% accuracy in identifying deep venous thrombosis. However, the positive predictive value of only 67% for CT venography suggests that sonography should be used to confirm the presence of isolated deep venous thrombosis before anticoagulation is initiated. In addition, interpretation of CT venography should be performed with knowledge of certain pitfalls.
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In the 1960s and 1970s, the diagnosis of pulmonary embolism was based on the catheter angiogram. Subsequently, the less invasive ventilation-perfusion scan was often obtained without catheter angiography [3]. Most recently, helical CT angiography is threatening to usurp this role, especially in patients with abnormal findings on chest radiographs [4,5,6,7,8,9,10]. Likewise, the diagnosis of deep venous thrombosis traditionally required conventional catheter venography [11]. Currently, noninvasive sonography has largely taken the place of contrast-enhanced venography [12, 13]. After the description of delayed helical CT venography after CT angiography in 1998, the optimal diagnostic evaluation of thromboembolic disease is in question once again [14].
Studies have already validated the accuracy of CT angiography for revealing pulmonary embolism above the subsegmental level [4,5,6,7,8,9,10]. Two additional studies have also shown that contrast-enhanced CT venography, with injection into the pedal veins, is comparable with contrast-enhanced venography for the diagnosis of deep venous thrombosis [15, 16]. However, to our knowledge, only two published studies have described the use of combined CT venography and CT angiography with a single upper extremity contrast injection. In the first study, Loud et al. [14] showed perfect correlation between the CT venographic results and sonographic results in five patients after CT angiography. In a recent larger prospective study, Loud et al. [17] again reported 100% correlation between CT venography and lower extremity sonography performed within 12 hr in 71 patients.
In the present study, we attempted to replicate the results of Loud et al. [17] using a retrospective study design. Helical CT venographic results of combined CT angiographic and CT venographic examinations were correlated with lower extremity sonographic findings for the identification of deep venous thrombosis when both examinations were performed within a 1-week interval.
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CT Angiographic and CT Venographic Protocol
Each examination was performed on a Somatom Plus 4 helical CT scanner
(Siemens Medical Systems, Munich, Germany). An IV injection of 120 mL of
iohexol (Omnipaque [300 mg organically bound I/mL]; Nycomed, Princeton, NJ)
was administered at 3 mL/sec via an 18- to 20-gauge antecubital catheter.
Breath-hold helical CT images were obtained with a 3-mm collimation and a 5-mm
table feed (pitch of 1.6) from the dome of the diaphragm to the aortic arch 28
sec after the start of the injection. After acquisition of the chest images
(usually 25-32 sec) and an additional 120-sec delay, helical CT images were
obtained with a 10-mm collimation and a 10-mm table feed (pitch of 1) from the
iliac crests to the knees. Images were routinely interpreted by subspecialty
trained thoracic radiologists in cine format on a Siemens work-station or
picture archiving and communication system (PACS). The primary criterion for
deep IV thrombosis included IV filling defects (Figs.
1A,1B,1C).
Additional secondary signs included venous dilatation, segmental
nonvisualization, wall enhancement, and perivenous and subcutaneous
infiltration.
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Sonographic Protocol
Each examination was performed on either a 128XP/10 (Acuson, Mountain View,
CA) or a Sonoline Elegra (Siemens Medical Systems) sonography unit and 5- or
7-MHz linear transducers. Transverse gray-scale imaging with and without
compression and longitudinal color and spectral Doppler imaging with
augmentation maneuvers were routinely performed from the upper common femoral
to the popliteal veins bilaterally. Images were obtained specifically of the
upper and lower common femoral vein; greater saphenous confluence; bifurcation
of the superficial and deep femoral veins; proximal, mid, and distal
superficial femoral and popliteal veins. If thrombus was identified in the
uppermost veins, additional evaluation was performed of the external iliac and
common iliac veins and inferior vena cava. If thrombus was seen in the
popliteal veins, additional evaluation of the calf veins was undertaken.
Examinations were either performed or checked by sonography-trained
radiologists. Criteria for deep venous thrombosis included noncompressible
veins, absence of vessel filling on color Doppler sonography, and either
absent or abnormal spectral Doppler waveforms (Figs.
1D,1E,1F).
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We reviewed the final dictated reports to determine the presence or absence of deep venous thrombosis on both helical CT venography and sonography. For the purposes of comparison, sonography was considered the gold standard [18]. Measures of sensitivity, specificity, positive and negative predictive values, and accuracy for helical CT venography versus sonography were calculated.
For each true-positive finding, the site of thrombus on both helical CT venography and sonography was recorded and compared for agreement. False-positive and false-negative helical CT venographic results were further analyzed. Tabulation of the time delay between these studies was performed. The helical CT venographic images were obtained and reviewed by the authors in an attempt to reach a consensus opinion regarding the potential cause or causes for the discrepancies.
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The final reports of the remaining 74 patients were evaluated for the presence or absence of deep venous thrombosis. On sonography, two patients had clots in the calf that had not been shown on CT venography; these cases were tabulated as "true-negatives" because the areas included for both techniques were free of thrombus. Sonography revealed deep venous thrombosis in nine (12%) of the 74 patients and no deep venous thrombosis in 65 (88%). Helical CT venography showed deep venous thrombosis in 12 patients (16%) and no deep venous thrombosis in 62 (84%). There were eight true-positive findings (11%), 61 true-negative (82%), four false-positives (5%), and one false-negative (1%). On the basis of these findings, the sensitivity of helical CT venography was 89%; specificity, 94%; positive predictive value, 67%; negative predictive value, 98%; and accuracy, 93%.
Of the eight cases with true-positive findings, agreement on the location of thrombus was excellent. Findings from five of the eight cases agreed perfectly, with the delay between examinations ranging from 1 to 2 days. The remaining three cases for which agreement was less than perfect included the following patients. The first patient was a 70-year-old man who showed minimal extension of thrombus into an adjacent common femoral vein on CT venography 5 days later (Figs. 1A,1B,1C). The second patient was an 89-year-old man who showed minimal resolution of thrombus in a common femoral vein on a follow-up sonography 1 day later. The third patient was a 76-year-old woman who displayed a large amount of deep venous thrombosis growth over a 3-day interval; the deep venous thrombosis was initially identified in the popliteal vein (Fig. 2A). After an arterial puncture, the patient developed a large adductor compartment hematoma and propagation of thrombus into the superficial femoral vein (Fig. 2B). Interestingly, pulmonary embolism was also detected in this patient on CT angiography, suggesting that the popliteal or superficial femoral vein thrombus may have even embolized to the lung (Fig. 2C). In addition, the other two patients with changes in the location of deep venous thrombosis also were found to have pulmonary embolism.
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Findings in four cases were four false-positive. One patient had a small nonocclusive clot on a single image of the common femoral vein that may have either developed or not been imaged at the time of sonography, 2 days earlier (Fig. 3). The other three patients had slight dilatation and vague filling defects seen on only a single image in two popliteal veins and one superficial femoral vein. On further review, the observers agreed that these focal defects might have represented volume averaging of valves (Fig. 4A,4B).
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The one false-negative finding was a case of extensive bilateral deep venous thrombosis involving the right common iliac to superficial femoral vein and the left common iliac to the popliteal vein on follow-up sonography 4 days later. On further review, the observers agreed that secondary signs of thrombus were present. These signs included subtle enhancement of the common femoral venous walls bilaterally, venous dilatation at this level, and both perivenous and subcutaneous edema (Fig. 5A). An additional finding that the observers thought might suggest extensive bilateral deep venous thrombosis was the prolonged arterial phase of enhancement (Fig. 5B).
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The time delay between helical CT venography and sonography was compared among the cases (Fig. 6). Of the examinations performed on the same day, all 14 had false-negative findings. Of the 24 examinations separated by a 1-day interval, one had a false-positive findings, three had true-positives, and 20 had true-negatives. There were 12 examinations separated by 2 days, of which two had false-positive findings, three had true-positives, and seven had true-negatives. Of the eight examinations separated by 3 days, one had true-positive findings and seven had true-negatives. There were five examinations separated by 4 days; one had a false-negative finding, one had a false-positive, and three had true-negatives. Five examinations were separated by 5 days: one had a true-positive finding and four had true-negatives. There were two and four additional false-negative findings for which examinations were separated by 6 and 7 days, respectively. The false-positive findings occurred after a delay of from 1 to 4 days. The false-negative finding occurred after a 4-day delay. No discordant findings were identified among examinations with a delay of more than 4 days between examinations.
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The cases with false-positive findings were divided into two groups depending on the appearance of the filling defect, well-defined and eccentric versus ill-defined and central. The first subgroup consisted of one patient with a well-defined eccentric 3-mm nonocclusive thrombus in a common femoral vein. Baldt et al. [16] described a similar nonocclusive thrombus in an eternal iliac vein that was identified on CT venography, but not on prospective sonography. Another possibility was that this abnormality developed after the sonographic examination 2 days earlier. As we showed with one of the patients with a true-positive finding, extensive thrombus extension can occur within 3 days. Surely, a 3-mm clot could form within 2 days. This discrepancy can be avoided in future studies by allowing less time between the examinations. The second subgroup consisted of three patients who had an ill-defined central filling defect seen on only single images (2 popliteal and 1 superficial femoral veins). The observers reviewed these images and thought each focal defect likely represented volume averaging of a valve and not thrombus. To better evaluate this possibility, narrower collimation, immediate follow-up sonography, or in vitro studies may be helpful.
The one case with a false-negative finding was misinterpreted for two reasons. The first reason was the large extent of the thrombus, which prevented any of the veins from enhancing normally. The second reason was its bilateral involvement, which caused the absence of a normal control vein for comparison. The prolonged arterial enhancement is interesting. This finding generally suggests two possible explanations. First, helical CT venography was performed too soon after CT angiography. Second, there is severe bilateral arterial insufficiency. This case illustrates a third possibility—that is, extensive bilateral deep venous thrombosis. Additional cases of extensive bilateral deep venous thrombosis will need to be identified to prove this hypothesis.
Additional methodologic differences were identified between our study and that of Loud et al. [17] that may have also contributed to the differing results. The largest of these differences was the use of prospective versus retrospective designs. The retrospective design has well-documented disadvantages, but this design also has certain advantages.
The largest disadvantage in our study was the failure to identify a large number of patients who had undergone both examinations on the same day. We were able to identify only 14 such patients. For these 14 patients, the CT venographic and sonographic results all agreed, but all revealed negative findings for deep venous thrombosis. It was for this reason that we chose to include patients with examinations separated by a larger time interval. We chose 1 week for three reasons. First, Holm et al. [19] suggested that even after adequate anticoagulation only 3% of the deep venous thrombosis cases resolve completely by 1 week. Second, Shah et al. [20] used a 1-week interval in assessing the accuracy of routine pelvic CT compared with lower extremity sonography for the detection of deep venous thrombosis. Lastly, if the delay between our studies allowed complete resolution or new development of thrombus, discrepancies should have been more apparent in the studies separated by longer intervals. This appeared to not be the case. Instead, discordant results were seen in cases separated by as few as 1 and as many as 4 days (Fig. 6). In fact, reducing the allowed delay between examinations to 4 days would have excluded 11 cases with true-negative findings. This would have only reduced the specificity from 94% to 93% and the accuracy from 93% to 92% without affecting the sensitivity or positive or negative predictive values.
An advantage of our retrospective design relates to the interpretation of the CT venographic studies. Instead of having the images interpreted by a small number of expert observers who were involved in the study design, we used four thoracic radiologists with differing levels of experience. They were unbiased because they were unaware of any potential study on this subject. We believe that this design more accurately depicts the situation seen in daily radiology practice. Furthermore, this design permitted the identification of the potential interpretive pitfalls.
A second methodologic difference between our study and that of Loud et al. [17] was in the CT venographic protocols. Loud et al. performed CT venography using a 5-mm collimation (10-mm collimation in 20 of 71 patients) and obtained images every 5 cm from the upper calves to the diaphragm 3.5 min after the start of contrast injection. For our protocol, we used a contiguous 10-mm collimation from the knees to the iliac crests 3 min after the start of contrast injection. Perhaps the use of a 5-mm collimation in our study might have prevented the mis-diagnosis of valves as thrombus. We suggest that prospective studies performed with different collimations, skip segments, time delays, and volumes of contrast material will help delineate an optimal CT venographic protocol.
One potential limitation that applies to both our study and that of Loud et al. [17] is the choice of sonography as the gold standard. It is possible that contrast-enhanced venography should remain the gold standard imaging study for the diagnosis of deep venous thrombosis. However, two limitations of contrast-enhanced venography support the use of sonography as the standard. First, contrast-enhanced venography is technically inadequate in 5-10% of the studies [21,22,23,24] and interobserver variability is reportedly 10% [25]. Second, sonography has been proven to be highly sensitive (89-100%) and specific (99-100%) for deep venous thrombosis when compared with contrast-enhanced venography of the thigh [26,27,28].
The long-term use of helical CT venography will be defined by the limitations of CT angiography. These limitations include a large range of reported sensitivity (53-100%) and specificity (81-100%) [29] and, in particular, a small but significant number of examinations that reveal false-negative findings, are technically inadequate, or both [4,5,6,7,8,9,10]. Patients who would otherwise be denied appropriate anticoagulation or would require another test before undergoing anticoagulation are the ones who might truly benefit from helical CT venography. In multiple studies, false-negative results range from 0.5% to 20% and technically limited examinations range from 0.7% to 10% [4,5,6,7,8,9,10]. In our series, only one CT angiographic examination (1.3%) was reported as technically not interpretable. However, 20 of these CT angiography reports (27%) described a degree of limitation because of respiratory motion. Therefore, until CT angiography shows consistently improved sensitivity, specificity, and insensitivity to patient motion, helical CT venography will likely have some role in the evaluation of patients with suspected pulmonary embolism.
Although the recent study of Loud et al. [17] indicates perfect agreement between helical CT venography and sonography in the diagnosis of deep venous thrombosis when experts interpret studies, we suggest a conservative approach to the use of helical CT venographic results at this time. We recommend that in patients without pulmonary embolism a positive finding on helical CT venography for deep venous thrombosis be confirmed on sonography before anticoagulation is undertaken. Furthermore, helical CT venography interpretation should be performed with a knowledge of certain pitfalls. These include ill-defined central venous filling defects seen on single images, which may represent valves, and prolonged arterial enhancement, which may represent extensive bilateral deep venous thrombosis.
Further prospective studies of CT angiography and helical CT venography, especially with the widespread implementation of multidetector helical CT, will be required to fully delineate the role of these techniques in the evaluation of patients with suspected pulmonary embolism.
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